1. Field of the Invention:
This invention relates in general to earth boring operations and in particular to an apparatus used to part the drill string in the drill collars to retrieve part of the drill collars should the drill string be stuck.
2. Description of the Prior Art:
In earth boring operations for oil and gas, a rotating drill string with a three cone drill bit is commonly used. The drill string is made up of drill pipe and drill collars. The bit usually is connected to a number of drill collars, typically 10 to 30 or more. The drill pipe is connected above the drill collars. Both the drill pipe and drill collars are hollow steel cylindrical members in sections of 30 feet or so. Fluid, normally a liquid, is pumped down the internal bore of the drill pipe and drill collars, out the drill bit, and back up the annular space between the drill string and the borehole wall. This circulates cuttings to the surface and cools the bit. The drill collars are much thicker than the drill pipe sections and are used to stabilize the bit and apply weight. Although thicknesses vary, typical drill pipe might have a wall thickness of 1/4 to 1/2 inch while the drill collars might have a wall thickness of 17/8 inch.
It is not uncommon for drill pipe to become stuck, usually due to a cave-in above the bit. In serious cases, the drill string cannot be rotated, pulled up, nor can circulation through the bit be established. Various techniques are known to free stuck pipe. In one technique, a free-point indicator can be lowered into the drill string on wire line. Tension is placed on the string while the free-point indicator is positioned at various points. It detects slight stretching of the pipe, which will not occur below the stuck point. Eventually this stuck point may be located.
Then, an explosive charge can be lowered on wire line and positioned near one of the drill pipe joints above the stuck point. The explosive is initiated while left-hand torque is applied to the string. The explosive impact in this "backoff" operation hopefully causes the joint to unscrew at that point. Then hydraulic jars, chemicals and other devices can be used to try to loosen the string or "fish" remaining in the hole.
At times "backoff" operations fail, and a cutter explosive charge is lowered on wire line to cut the drill pipe above the stuck point to salvage as much pipe as possible. The cutter charge is a radially directed shaped charge having sufficient energy to sever the drill pipe. All of the drill collars and the remaining drill pipe below where cut will be abandoned in the hole. A new hole may be started, or the old hole may be directionally drilled from a new angle from above the abandoned drill pipe.
If the stuck point is at the bit or in the collars, at present there is no known way to salvage the drill collars if a "backoff" operation fails. Cutter charges may be sufficient to cut the drill pipe, but no known charges have sufficient energy to part a drill collar. Consequently all of the expensive drill collars will ordinarily be lost since the operator will be forced to cut the drill pipe, even though the stuck point may be far below.